Human growth hormone (hGH) is a hormone that participates in the regulation of human growth and development. Growth Hormone (herein after “GH”), also known as somatotrophin, represents a class of proteinaceous hormones produced and secreted by the somatotropic cells of the anterior pituitary. Secretion of GH is stimulated by the growth hormone releasing hormone (GHRH) from the hypothalamus and suppressed by somatostatin. This pituitary hormone exhibits a number of biological effects including somatogenesis, lactation, activation of macrophages, insulin-like and diabetogenic effects among others (Chawla, R, K. (1983) Ann. Rev. Med. 34, 519; Edwards, C. K. et al. (1988) Science 239, 769; Thorner, M. O., et al. (1988) J. Clin. Invest. 81, 745). Human growth hormone is a member of a family of homologous hormones that include placental lactogens, prolactins, and other genetic and species variants of GH. GH regulates the secretion of Insulin-like growth factor (IGF-1, formerly known as somatomedin C), among other peptide hormones known collectively as somatomedins, which accounts for most of its biological activity.
A number of diseases and disorders are associated with the deficiency of GH. A deficiency can be congenital, acquired in childhood or in adult life, and can be partial or complete. In some cases, the deficiency is transient, but more commonly is permanent, and may occur in association with deficiencies of other pituitary hormones. Growth hormone deficiency in children leads to dwarfism, growth failure or short stature. Deficiency in adults is rare, but symptoms can include diminished body mass and poor bone density, and a number of psychological symptoms. Other hormonal or glandular disorders frequently coincide with deficiency of growth hormone.
Stimulating the increase in height in childhood is the most widely known effect of GH, and appears to function by at least two mechanisms: GH directly stimulates division and multiplication of chondrocytes of cartilage, and GH also stimulates production of IGF-1. IGF-1 has growth-stimulating effects on a wide variety of tissues. Additional IGF-1 is generated within target tissues, making it apparently both an endocrine and an autocrine/paracrine hormone. IGF-1 also has stimulatory effects on osteoblast and chondrocyte activity to promote bone growth.
Human growth hormone (hGH) plays a key role in somatic growth through its effects on the metabolism of proteins, carbohydrates and lipids. In addition to its effects on somatic growth, hGH has been shown to stimulate blood cells in vitro (Derfalvi et al., 1998; Merchav et al; 1988), to increase erythrocytes and hemoglobin counts (Valerio et al., 1997; Vihervuori et al., 1996), to enhance both proliferation and Ig production in plasma cell lines (Kimata and Yoshida, 1994) and to stimulate CD8+ cell counts and, to a lesser extent CD4+ cell counts (Geffner, 1997).
Injectable forms of GH have been marketed for GH deficiency in children and adults, Turner Syndrome, Prader-Willi Syndrome, and children small for gestational age. In addition, it has seen use in the battle against aging and for weight management, as well as the mobilization of cells capable of regenerating hematopoiesis in the peripheral blood.
The 22 kDA molecular weight of hGH is well below the threshold value for kidney filtration of about 70 kDa (Caliceti (2003) Adv Drug Deliv Rev 55:1261-1277), which contributes to the serum half-life of native hGH being less than 20 minutes in humans. Thus, commercial preparations of hGH must be dosed daily to achieve clinical benefit. A sustained-release form of GH, Nutropin Depot (Genentech and Alkermes) was approved by the FDA in 1999, allowing for fewer injections (every 2 or 4 weeks instead of daily); however, the product was discontinued in 2004.
Chemical modifications to a therapeutic protein can modify its in vivo clearance rate and subsequent serum half-life. One example of a common modification is the addition of a polyethylene glycol (PEG) moiety, typically coupled to the protein via an aldehyde or N-hydroxysuccinimide (NHS) group on the PEG reacting with an amine group (e.g. lysine side chain or the N-terminus). However, the conjugation step can result in the formation of heterogeneous product mixtures that need to be separated, leading to significant product loss and complexity of manufacturing and does not result in a completely chemically-uniform product. Also, the pharmacologic function of GH may be hampered if amino acid side chains in the vicinity of its binding site become modified by the PEGylation process. Other approaches include the genetic fusion of an Fc domain to the therapeutic GH protein. Conjugation of the Fc domain increases the size of the therapeutic protein, hence reducing the rate of clearance through the kidney. Additionally, the Fc domain confers the ability to bind to, and be recycled from lysosomes by, the FcRn receptor, which results in increased pharmacokinetic half-life. Unfortunately, the Fc domain does not fold efficiently during recombinant expression, and tends to form insoluble precipitates known as inclusion bodies. These inclusion bodies must be solubilized and functional protein must be renatured from the misfolded aggregate, a time-consuming, inefficient, and expensive process. Accordingly, there remains a need for growth hormone compositions that can increase the half-life and can be administered less frequently, but are safer and less complicated and costly to produce.